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Hi cottontail Its more the humidity with the heat that causes the problem. Make sure you water sufficiently for a start and did you mulch well? Both help the plant to be more resistant. The worst leaves can be cut off and destroyed and remove any debris from around the plants. Vitax Organic 2 in 1 is good to control mildew but I think you'll find that the flowers will be fine anyway just the plants look a mess.
I just pick off any ropey looking leaves hct and put them in the green waste collection rather than my own compost. As addict says sometimes it's humidity that causes issues as there's always a bit of congestion around the base of the plants once they get going, especially if you have them in pots.
I had powdery mildew on my new monarda and sprayed it with a milk mix (1 part milk 8 parts water I used 2%!!!) that was 1 month ago no more mildew!!! You can also use bicarbonate of soda I believe it's 1 tablespoon + I teaspoon of washing up liquid in 1 gallon of water( mug mum swears by it for established plants) spray every 2 weeks